Phase Transitions in [M(NH₃)₆] (XY₄)₂ Compounds

Abstract The group‐theoretical analysis of the phase transitions in the ionic crystals [Ni(NH 3 ) 6 ] (ClO 4 ) 2 , [Mg(NH 3 ) 6 ](ClO 4 ) 2 , and [Ni(NH 3 ) 8 ](BF 4 ) 2 , accompanied by the lowering of the space groups from Fm3m to P2 1 /b is carried out. It is shown that the symmetry reduction is associated with the two‐dimensional irreducible representation τ 10 of the star { k 10 }, which corresponds to the point X of the f.c.c. Brillouin zone. The analysis of the lattice normal modes in Fm3m phase allowed one to specify the structural changes created at the phase transition point. The monoclinic crystal structure with the space group P2 1 /b may arise as a result of translational displacements of all ions in the plane parallel to the unique symmetry plane. The relative orientations of anions become also fixed.

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